1. Field of the Invention
The present invention relates to a Mach-Zehnder type modulator which is an important device in an optical transmission system and an optical information processing system, and a method of driving the modulator. More particularly, this invention relates to a Mach-Zehnder type modulator which can prevent the generation of positive wavelength chirping even when a modulation electric signal is applied only to one phase modulator section, and a method of driving this modulator.
2. Description of the Related Art
Conventionally, a waveguide type optical device, such as a semiconductor laser or an optical modulator, is considered as one of important devices in a fast optical transmission system and optical information processing system, and the research and development on such optical devices become active.
In accordance with the recent improvement on the speed and transmission length of optical transmission systems, however, the problem of the conventional direct modulation system by a semiconductor laser is surfaced. In the direct modulation system by a semiconductor laser, wavelength chirping occurs at the time of modulation, thus degrading the waveform after fiber transmission. The faster the signal transfer speed is and the longer the signal transmission length becomes, the more prominent this waveform degradation becomes.
The waveform degradation is a significant problem particularly in a system using a 1.3 .mu.m dispersion fiber. This is because even if a light source with a low fiber transfer loss, e.g., a light source whose wavelength lies in a 1.55 .mu.m band, is used to extend the transmission length, the transmission length is limited by the chirping-originated dispersion restriction. This problem can be overcome by employing an external modulation system which causes a semiconductor laser to emit a constant optical output and modulate the emitted light from the semiconductor laser with an optical modulator other then the semiconductor laser. In this respect, the development on external optical modulators which externally modulate the emitted light from a semiconductor laser is recently in progress.
Such external optical modulators include a type which uses a dielectric like LiNbO.sub.3 and a type which uses a semiconductor like InP or GaAs. There is an increased expectation for the semiconductor optical modulators, among those different types of external optical modulators, which can be designed into an integrated circuit with another optical device like an optical amplifier or an electronic circuit like an FET and whose miniaturization and consumed power reduction can easily be accomplished. Typical type of semiconductor optical modulators are an absorption type optical modulator and a Mach-Zehnder (MZ) type modulator. The absorption type optical modulator utilizes the effect of shifting the light absorption end toward the long wavelength side by applying an electric field to a semiconductor as in the Franz-Keldish effect of a bulk semiconductor or a Quantum-Confined Stark Effect of a multiple quantum wells (MQW) structure. The MZ modulator utilizes the effect of changing the refractive index by applying an electric field to a semiconductor as in the electro-optical effect (Pockels effect) of a bulk semiconductor or the Quantum-Confined Stark Effect of an MQW structure.
The absorption type optical modulator, which is one of the external modulation type semiconductor optical modulators, can achieve significantly small wavelength chirping as compared with the semiconductor laser direct modulation type, but cannot achieve zero wavelength chirping. The MZ modulator can in principle achieve zero or negative wavelength chirping, and is thus expected to be a modulator for the future super fast long distance optical transmission. Proposed as one example of a semiconductor MZ modulator is a high mesa type MZ modulator which has an InGaAs/InAlAs multiple quantum wells as a waveguide layer (see Sano et al., Proceedings of the 1993 IEICE (the Institute of Electronics, Information and Communication Engineers) Spring Conference, Book 4, pp. 4-187 (Lecture No. C-151) and Sano et al., OFC/IOOC '93 Technical Digest, Thursday Afternoon, pp. 215-217, 1993).
In this high mesa type MZ modulator, for example, 6.5 nm InGaAs well layer and 6.0 nm InAlAs barrier layer constitutes MQW with respect to the incident-light wavelength of 1.55 .mu.m, and the band-gap wavelength is set to 1.45 .mu.m. The waveguide layer has 30-period MQW's of InGaAS/InAlAs and has a device length of 1.2 mm, and two phase modulator sections to which an electric field is applied have the same length (e.g., 0.5 mm) and the same structure. For this device, the modulation voltage (half-wavelength voltage) with respect to the incident-light having a wavelength of 1.55 .mu.m is 4.2 V, an extinction ratio is 13 dB and the fiber insertion loss is 12 dB.
An MZ modulator, fabricated by using a semiconductor material, particularly, one having an MQW structure as a refractive-index variable medium, has a size of approximately 1 mm, which is extremely small as compared with the size of an MZ modulator (10 mm) which uses a dielectric material like LiNbO.sub.3.
As a method of driving a semiconductor MZ modulator, a driving method which involves a ridge waveguide type MZ modulator having InGaAs/InP MQW's as a waveguide layer was reported (Ishizaka et al., Proceedings of the 1994 IEICE (the Institute of Electronics, Information and Communication Engineers) Autumn Conference, Book 1, p. 173 (Lecture No. C-173)). This report describes the results of analysis on wavelength chirping by using a ridge waveguide type MZ modulator to modulate light by a half modulation system (single electrode driving system) or a push-pull modulation system.
The half modulation system will be described first. FIG. 1 is an exemplary diagram showing a modulation method based on the half modulation system which uses a ridge waveguide type MZ modulator. This modulator device has an incident-light waveguide path 31 for waveguiding incident light, a splitter 32 for splitting the waveguided light to two, two waveguide paths (first waveguide path 33a and second waveguide path 33b) branched from the splitter 32, a mixer 34 for synthesizing the output lights from the two waveguide paths, and an outgoing-light waveguide path 35 which waveguides the synthesized light. A first phase modulator section 21 and a second phase modulator section 22 are respectively formed in the first waveguide path 33a and the second waveguide path 33b. The second phase modulator section 22 in the second waveguide path 33b is electrically grounded, while the first phase modulator section 21 in the first waveguide path 33a is connected to a signal source 23 so that a voltage corresponding to an electric signal can be applied to the first phase modulator section 21 from the signal source 23.
FIG. 2A is a graph which shows a modulation electric signal to be applied to the first phase modulator section 21 by the half modulation system by plotting the input signal voltage on the vertical scale and time on the horizontal scale, FIG. 2B is a graph which shows the intensity of output light by plotting the output light intensity on the vertical scale and time on the horizontal scale, and FIG. 2C is a graph showing a frequency change of output light by plotting a frequency change on the vertical scale and time on the horizontal scale. To modulate light by the half modulation system using the modulator designed as shown in FIG. 1, a voltage of V.pi. (0-t.sub.1, t.sub.4 -t.sub.5) or a voltage of 0 (t.sub.2 -t.sub.3) from the signal source 23 is applied as a modulation input signal the first phase modulator section 21 in the first waveguide path 33a, as shown in FIG. 2A. It is apparent from FIG. 2B that when the voltage of V.pi. is applied to the first phase modulator section 21, the light OFF state is acquired, and when the voltage of 0 is applied to the first phase modulator section 21, the light ON state is acquired. While the light incident to the incident-light waveguide path 31 is waveguided through the splitter 32, the first and second waveguide paths 33a and 33b and the mixer 34, the light is modulated by the modulation input signal and the modulated light comes out from the outgoing-light waveguide path 35.
As shown in FIG. 2C, however, this half modulation system generates a positive chirping component 24 at the point of the light ON (t.sub.1 -t.sub.2) or the light OFF (t.sub.3 -t.sub.4). The positive chirping is a time-dependent frequency change such that the frequency change of a light wave shows a positive value when the light output intensity increases (light ON time) and this frequency change shows a negative value when the light output intensity decreases (light OFF time). When such positive chirping occurs, the waveform of the light output signal spreads and is degraded as it propagates.
The push-pull modulation system will now be described. FIG. 3 is an exemplary diagram showing a modulation method based on the push-pull modulation system which uses a ridge waveguide type MZ modulator. The device shown in FIG. 3 differs from the one in FIG. 1 only in that a first signal source 23a and a second signal source 23b are respectively connected to the first phase modulator section 21 and the second phase modulator section 22. To avoid the redundant description, therefore, like or same reference numerals are given to those components in FIG. 3 which are the same as the corresponding components shown in FIG. 1. FIG. 4A is a graph which shows a modulation electric signal to be applied to the first phase modulator section 21 and the second phase modulator section 22 in the push-pull modulation mode by plotting the input signal voltage on the vertical scale and time on the horizontal scale, FIG. 4B is a graph which shows the intensity of output light by plotting the output light intensity on the vertical scale and time on the horizontal scale, and FIG. 4C is a graph showing a frequency change of output light by plotting a frequency change on the vertical scale and time on the horizontal scale.
To modulate light by the push-pull modulation system using the modulator designed as shown in FIG. 3, signal voltages of the opposite phases are applied as modulation input signals to both phase modulator sections 21 and 22. As shown in FIG. 4A, for example, a voltage of V.pi. (0-t.sub.1, t.sub.4 -t.sub.5) or a voltage of (V.pi./2) 36 (t.sub.2 -t.sub.3) from the first signal source 23a is applied to the first phase modulator section 21 in the first waveguide path 33a. A voltage of 0 (0-t.sub.1, t.sub.4 -t.sub.5) or a voltage of (V.pi./2) 37 (t.sub.2 -t.sub.3) from the second signal source 23b is applied to the second phase modulator section 22 in the second waveguide path 33b. It is apparent from FIG. 4B that when the voltage of V.pi. and the voltage of 0 are respectively applied to the first phase modulator section 21 and the second phase modulator section 22, the light OFF state is acquired, and when the voltage of (V.pi./2) is applied to both the first and second phase modulator sections 21 and 22, the light ON state is acquired. While the light incident to the incident-light waveguide path 31 is waveguided through the splitter 32, the first and second waveguide paths 33a and 33b and the mixer 34, the light is modulated by the modulation input signal and the modulated light comes out from the outgoing-light waveguide path 35.
As shown in FIG. 4C, this push-pull modulation system generates a negative chirping component 25 at the point of the light ON (t.sub.1 -t.sub.2) or the light OFF (t.sub.3 -t.sub.4). The negative chirping is a time-dependent frequency change such that the frequency change of a light wave shows a negative value when the light output intensity increases (light ON time) and this frequency change shows a positive value when the light output intensity decreases (light OFF time). When such negative chirping occurs, the degradation of a signal waveform which is caused by the dispersion of the optical fiber can be suppressed by the waveform compression.
Since this push-pull modulation system needs different voltages to be applied to both phase modulator sections 21 and 22, however, the drive system for the electric circuit, the signal transmission path and the like which transmit an electric signal becomes complicated. It is therefore difficult to adapt the push-pull type MZ modulator for use in an actual optical transmission system.